A ‘PI’ attenuator circuit such as one shown in FIG. 1 may be employed to control the amplitude levels of a modulation signal. It is an optical network, for example, such a circuit may be used to control the amplitude signal applied to modulate the output of a laser beam. Properly biased, the diodes 102, 104 may behave in the circuit as current-sensitive resistors with an impedance that varies according to the current injected from the biasing resources VC and V+.
Shunt arms 126 and 128 shunt the input RF signals and output RF signals respectively to ground via RC circuits 104, 124, and 120, 122, which provide a DC bias potential on the diodes 106, 112 while passing non-DC bias components to ground. Likewise, the voltage divider circuits comprising resistors 102, 116, and 114 bias diodes 108 and 110. The diodes 106, 112, 108, 110 are typically PIN (P-type, Insulator, N-type) diodes. An input RF signal is attenuated by the circuit with the actual attenuation varying according to the biasing current provided by Vc and V+. The circuit thus provides a tunable RF attenuator.